Kinetic Features of the Platinum Catalyzed Hydrolysis of Sodium Borohydride from ¹¹B NMR Measurements

A full kinetic analysis of the platinum catalyzed hydrolysis of sodium borohydride (NaBH₄) in alkaline media has been performed using ¹¹B NMR (nuclear magnetic resonance) spectroscopy with a Pt/C 5 wt % commercial powder as catalyst. By fitting the NMR data by least-square regression techniques, the rate constants of platinum catalyzed borohydride hydrolysis have been evaluated. Within the investigated [borohydride]/[catalyst] molar ratio of 200−1500, the rate law has been found to be of first-order in catalyst and zero order in borohydride. Whereas no reagent-isotope kinetic effect is observed in the NaBD₄/H₂O reaction system, the hydrolysis of NaBH₄ in deuterated water shows a significant solvent-kinetic isotope effect. In both cases, however, ¹¹B NMR analysis indicates that the main reaction product is the tetrahydroxyborate species (D)_nH_4-nBO₄^- (n = 1, 2, 3, 4) followed by a minor amount of the partially scrambled BH₃D^- species while the hydrolysis-intermediates H_nB(OD)_4-n^- (n = 1, 2, 3) species are not detectable during all the reaction time. These results suggest that, differently from the Pd catalyzed/borohydride(borodeuteride) hydrolytic process, the hydrogen/deuterium exchange is slower than hydrolysis and that the rate-determining step of the overall process is the formation of the monohydroxy-borohydride intermediate BH₃OH^-. The activation energy of the overall process has been also evaluated by ¹¹B NMR rate measurements taken at different temperatures.